In hydrodesulfurization, the overhead vacuum distilled sour gas oils and naphthas employed as feedstocks are prone to produce material that deposits and accumulates upon the surfaces of hydrodesulfurization catalysts and also equipment such as heat transfer surfaces and the like, contacted therewith resulting in the fouling of such catalysts and equipment. In normal, continuous use, for example, the heat exchangers used in such equipment suffer gradually increasing losses in efficiency, heat transfer, pressure drop, and throughout owing to deposition of material on the inner surfaces thereof. Consequently, hydrodesulfurization units must be periodically shut down and the deposite removed therefrom. Such fouling of equipment, such as heat exchangers, furnaces, pipes, chambers, auxiliary equipment, and the like, is costly by reason of the loss of production time and the man hours required for disassembly, cleaning and reassembly of such unit process equipment components. The equipment is usually fabricated of carbon steel, stainless steel, or aluminum.
The catalysts used in hydrodesulfurization are also subject to fouling and contamination, and the equipment must be periodically shut down and the catalyst replaced on this account which adds significantly to operational costs.
The fouling is generally attributed to the presence of unstable components, such as oxidized derivatives of hydrocarbons, the inorganic impurities present in hydrocarbon fractions, organosulfur compounds, the presence of olefinic unsaturated hydrocarbons or their polymeric derivatives, or the like. Thus, characteristically naphthas used as hydrodesulfurization feedstocks contain minor amounts of readily oxidized and oxidizable hydrocarbon constituents. Furthermore, almost all such feedstocks contain amounts of sulfur, dissolved oxygen, and metals, in a free and/or chemically combined state. Chemical and/or thermal treatment of such feedstocks can result in the polymerization of the olefinic substitutes. The fouling deposits themselves are typically and principally polymerization products and are characteristically black in color. Some are initially gummy masses which convert to coke-like masses at elevated temperatures. Inorganic portions of such deposits frequently contain components, such as silica, iron-oxides, sulfur oxides, iron sulfides, calcium oxide, magnesium oxide, inorganic chloride salts, sodium oxide, alumina, sodium sulfate, copper oxides, copper salts, and the like. These deposits are not readily solubilized by common organic solvents and these deposits are distinguishable from the corrosion and sludge formation sometimes occuring in finished products. Conventional antioxidants, stabilizing chemicals, and the like are characteristically relatively ineffective as antifoulants.
The use of certain organophosphorous compounds as antifoulant additives to mineral hydrocarbon mixtures employed as refinery feedstocks undergoing a heat treatment (indluding distillation) has heretofore been proposed. See, for examples, Fierce et al U.S. Pat No. 2,899,387; Cyba U.S. Pat. No. 3,017,357; Newkirk et al U.S. Pat. No. 3,261,774; Koszman U.S. Pat. No. 3,531,394; Gillespie et al U.S. Pat. No. 3,558,470; Gillespie et al U.S. Pat. No. 3,645,886; Wolff et al U.S. Pat. No. 3,647,677; and the like. However, hydrodesulfurization is, as those skilled in the art appreciate, readily distinguishable from other petroleum processing generally by reason of the special conditions, sequences, equipment, catalysts, and coreactants (especially hydrogen) employed. Additives added to upstream precursers of the typical feedstocks used in hydrodesulfurization are typically substantially removed during the upstream processing steps employed (which result in the naphtha distillates used as the hydrodesulfurization feedstocks).
So far as known to us, no one has heretofore ever employed mono and di phosphate or phosphite esters, thioesters, and amine salts thereof, as antifoulant additives for hydrodesulfurization feedstocks. Such phosphorous esters, thioesters and amine salts thereof, have now been found characteristically to display surprising and very useful antifoulant activity in hydrodesulfurization. Not only do these materials inhibit and suppress, and even prevent, fouling when in such feedstocks but also they unexpectedly appear to reduce the fouling in previously used and fouled hydrodesulfurization equipment. Furthermore, these additives also inhibit, suppress and prevent fouling of the catalysts involved without poisoning or adversely affecting catalyst properties, which is a surprising and unusual effect. Such additives in combination can sometimes be considered to be arguably synergistic in some of these effects as those skilled in the art will appreciate. The art of reducing and regulating fouling in hydrodesulfurization without adversely affecting catalysts is very complex and the reasons why a particular antifoulant system works to reduce and regulate fouling without itself harming catalysts are not now known or understood.